Electron-spin resonance of a complexPb+(6p1)defect in alkali halides

The electron-spin-resonance spectra are analyzed of a ${\mathrm{Pb}}^{+}$ defect produced by a long x-ray irradiation below 150 K in ${\mathrm{Pb}}^{2+}$-doped KCl and RbCl. At liquid-helium temperatures the main symmetry axis of the center is tipped a few degrees away from in a {110} plane. At higher temperatures the center exhibits a librational motion which averages out the tipping effects but leaves the spectra with a small orthorhombic component. A model is proposed which is in agreement with these facts and with the behavior of the hyperfine components: A substitutional ${\mathrm{Pb}}^{+}$ possessing weak molecular bonds with two adjoining substitutional ${\mathrm{Cl}}^{-}$ ions is perturbed by an interstitial ${\mathrm{Cl}}^{-}$ ion in a next-nearest interstitial tetrahedral site. The analysis of the hyperfine data confirms a general property of the $n{p}^1\mathrm{}(n=4,5,6\mathrm{})\mathrm{}$ heavy metal ions and atoms: The electron-spin density at the nucleus is large and negative.